Germanium is a widely used optical material for long-wave infrared (LWIR) applications due to its high refractive index, excellent mechanical strength, and strong transmission in the 8–14 µm wavelength range. When selecting germanium for LWIR imaging systems or optical windows, customers often ask whether N-type or P-type germanium provides better infrared transmission.
Understanding IR Transmission in the LWIR Band
The infrared transmission performance of germanium in the LWIR range is influenced primarily by free carrier absorption, which is directly related to the carrier concentration (resistivity) of the material rather than the dopant type alone.
N-Type Germanium
- Doped with elements such as phosphorus, arsenic, or antimony
- Free electrons contribute to free carrier absorption in the LWIR region
- High-resistivity N-type germanium can achieve excellent LWIR transmission
- Heavily doped (low-resistivity) material will show reduced transmission due to increased absorption
P-Type Germanium
- Doped with elements such as boron, gallium, or indium
- Free holes also contribute to free carrier absorption
- Commonly available in very high-resistivity grades, which minimizes absorption in the 8–14 µm range
- Frequently used in LWIR optics due to material availability and industry standardization
What Really Determines LWIR Transmission Performance
For LWIR optical windows and imaging lenses, resistivity is the dominant factor, not whether the germanium is N-type or P-type.
Key factors affecting LWIR transmission include:
- Material resistivity (Ω·cm)
- Optical thickness
- Surface quality and polish
- Anti-reflection (AR) coating performance in the 8–12 µm or 8–14 µm band
When properly specified, both N-type and P-type germanium can deliver high transmission suitable for demanding LWIR applications.
Suitability for LWIR Imaging and Optical Windows
- LWIR Imaging Systems: High transmission improves detector sensitivity and image quality. High-resistivity germanium—most commonly P-type—is widely used in thermal cameras and infrared imaging systems.
- LWIR Optical Windows: For protective windows and system enclosures requiring efficient LWIR transmission, high-resistivity germanium with appropriate AR coatings provides excellent optical performance and environmental durability.
Conclusion
P-type germanium is often preferred for LWIR imaging and optical window applications because it is readily available in high-resistivity grades, which reduces free carrier absorption in the 8–14 µm range. However, when resistivity is properly controlled, both N-type and P-type germanium can offer comparable LWIR transmission performance.
At SOT, we supply high-quality germanium optics tailored for LWIR applications, with material selection, resistivity control, and coating design optimized to meet your system requirements.